log/slog: rename and remove files

log/slog: rename and remove files

- Remove the norace_test.go files, moving their contents elsewhere.

- Rename the internal/testutil package to internal/slogtest.

- Remove value_unsafe.go, moving its contents to value.go.

Updates golang/go#56345.

Change-Id: I2a24ace5aea47f7a3067cd671f606c4fb279d744
Reviewed-on: https://go-review.googlesource.com/c/go/+/478197
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>